This invention relates to a projection exposure apparatus and a device manufacturing method, in which a pattern of a first object such as a mask or a reticle is imaged on and transferred to a second object such as a photosensitive substrate or a wafer, for example. The present invention is suitably usable in a lithographic process for manufacture of various devices such as a semiconductor device (e.g., IC or LSI), an image pickup device (e.g., CCD), a liquid crystal panel or a magnetic head, for example.
The integration of a semiconductor device such as IC or LSI has increased more and more, and a microprocessing technique for the semiconductor device has advanced greatly. As regards a projection exposure apparatus which is the main part of the microprocessing technique, there are (i) a unit magnification exposure apparatus (mirror projection aligner) wherein, by using a mirror optical system of unit magnification having an arcuate shape exposure region, a mask and a photosensitive substrate are exposed in a single exposure operation while they are scanned simultaneously, and (ii) a reduction projection exposure apparatus (stepper) wherein, by using a dioptric system, an image of a mask pattern is formed on a photosensitive substrate and the photosensitive substrate is exposed in a step-and-repeat method.
In these projection exposure apparatuses, the stepper has an advantage over the mirror projection aligner in the point of resolving power and registration precision. Recently, a step-and-scan type (scanning) projection exposure has been proposed, by which a high resolving power is attainable while, on the other hand, the picture plane size can be enlarged.
A step-and-scan type exposure apparatus has a slit-like exposure region, and, in regard to each shot, a scanning exposure (scan of the slit) is performed. As the scanning exposure of one shot is completed, the wafer is moved to a subsequent-shot exposure position, and the scanning exposure of the subsequent shot is initiated. In this manner, the stepwise motion to a subsequent shot and the scan for exposure of that shot are repeated, by which the exposure of the whole wafer is accomplished.
In projection exposure apparatuses, improvements of resolution have been attempted to attain formation of a pattern image of a size of 0.5 micron or smaller. Specifically, enlargement of the numerical aperture (NA) of a projection lens or shortening of the wavelength of exposure light have been proposed. Generally, as the exposure wavelength is shortened, the transmission factor of a glass material decreases, and only a few types of glass materials can be used in a projection exposure apparatus. As the types of glass materials are limited, the correction of chromatic aberration of the projection optical system becomes difficult to attain, so that the bandwidth of exposure light has to be narrowed to a level with which the influence of chromatic aberration can be disregarded. For example, in a projection optical system to be used with light of a wavelength region of 300 nm or less, usable glass materials are only quartz and fluorite. Thus, generally a laser is used as an exposure light source. In lasers of such type, an excimer laser has a very high energy so that a high throughput can be expected. Therefore, it is widely used as a short-wavelength light source in a projection exposure apparatus.
Since excimer lasers are pulse lasers, use of an excimer laser in a projection exposure apparatus as a light source involves some problems. Particularly, in a mirror projection aligner or a scan type projection exposure apparatus such as a step-and-scan type exposure apparatus, there is a possibility that, depending on the scan speed or the pulse light emission timing, exposure non-uniformness is produced upon a mask surface or a wafer surface.
Japanese Laid-Open Patent Application, Laid-Open No. 158449/1985 shows a projection exposure apparatus wherein such exposure non-uniformness due to the pulse light emission can be reduced. More specifically, the light intensity distribution to be produced upon surface to be illuminated, with respect to a scan direction, is made substantially into an isosceles triangle or trapezoid shape, by which the synchronization precision for the scan speed and the pulse light emission is loosened. The control for making the light intensity distribution in the scan direction into a desired shape, is attained by inserting a light attenuating element such as an ND filter into the light path of an illumination system. The shape of the light intensity distribution is not limited to an isosceles triangle or trapezoid. Any shape may be used, provided that the dispersion of exposure amount when distributions are superposed is small.
Japanese Laid-Open Patent Application, Laid-Open No. 230949/1995 shows an improved projection exposure apparatus wherein, in a plane which contains a scan direction of a surface to be illuminated and an optical axis of an illumination optical system, chief rays of the flux of illumination light are collected at a position spaced by a predetermined distance from the surface to be illuminated, along the optical axis direction, such that a tilt region due to blur is produced in the light upon the surface illuminated. By making the light intensity distribution in the tilt region into a desired shape, an illumination system and a projection exposure apparatus having the same, with which the decrease of illumination efficiency is prevented while the non-uniformness of exposure amount is reduced, are accomplished. In accordance with this proposal, in a projection exposure apparatus, the shape of the light intensity distribution in the scan direction and upon the surface to be illuminated, can be determined so that the dispersion of integrated exposure amount attributable to unevenness of scan speed or variation in timing of pulse light emission is reduced to minimum.
However, generally, a projection exposure apparatus is used in various illumination conditions. If the illumination condition changes, for example, when an effective light flux is restricted, the shape of the tilt region changes which may cause reduction in width of blur and the uniformness of integrated exposure amount upon the surface being illuminated may be degraded.
FIG. 1 illustrates exposure non-uniformness, in scan exposure using a pulse emission type laser light source, to be produced in a scan direction where the light intensity distribution in the scan direction is improper. The axis of ordinate denotes the light intensity, and the axis of abscissa denotes the position coordinates in the scan direction. Because of pulse emission, the exposure is discrete. Light intensity distributions produced by respective pulses are superposed with a shift, by which the whole light intensity distribution (integrated exposure amount distribution) is produced. If the relationship between the shift amount and the light intensity distribution is not proper, there may occur periodic exposure unevenness such as shown in FIG. 1.
FIG. 2 shows a case where the relation between the shift amount and the light intensity distribution is proper. It is seen that the integrated exposure amount is kept uniform.
It is an object of the present invention to provide an improved projection exposure apparatus and/or a device exposure method using the same, by which, in scanning exposure using a laser light source of pulse emission type, for example, the light intensity distribution in a scan direction, upon a surface to be illuminated, can be made into a desired shape regardless of the size and/or the shape of light upon a secondary light source plane, such that a uniform integrated exposure amount distribution can be produced.
Here, the secondary light source refers to an effectual size of a light source image to be formed for illumination of an article to be illuminated. For example, it may be a light source image formed at a light exit surface of an optical integrator included in an illumination system, or a stop which serves to determine the size of the light source image as a whole.
In accordance with an aspect of the present invention, there is provided a projection exposure apparatus for transferring, by projection exposure, a pattern of a first object onto a second object while scanning the first and second objects in synchronism with each other, said apparatus comprising: an illumination optical system having secondary light source forming means for forming a secondary light source and a slit disposed with a space from the first object or a plane conjugate thereto, said illumination optical system being operable to illuminate the first object with light supplied from a light source of a pulse light emission type and having a slit-like irradiation region defined through said slit on a light path of said illumination optical system; a projection optical system for projecting the pattern of the first object onto the second object; and control means, wherein, when R is an effective radius of the secondary light source, d is an optical distance from the slit to the first object or the conjugate plane thereof, f is a focal length of an optical member disposed between the secondary light source and the slit, F is a pulse emission frequency of the light source, and V is a scan speed of the first object in the scan exposure, said control means is operable to make uniform an integrated exposure intensity upon the first object and in the scan direction in accordance with a change in any parameter of R, d, f, F and V, by changing at least one of the remaining parameters.
In accordance with another aspect of the present invention, there is provided a projection exposure apparatus for transferring, by projection exposure, a pattern of a first object onto a second object while scanning the first and second objects in synchronism with each other, said apparatus comprising: an illumination optical system having secondary light source forming means for forming a secondary light source and a slit disposed with a space from the first object or a plane conjugate thereto, said illumination optical system being operable to illuminate the first object with light supplied from a light source of a pulse light emission type and having a slit-like irradiation region defined through said slit on a light path of said illumination optical system; a projection optical system for projecting the pattern of the first object onto the second object; and control means for controlling an exposure parameter so that an integrated exposure intensity upon the first object and in the scan direction in accordance with a change in size of the secondary light source.
In accordance with a further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: transferring, by projection exposure, a pattern of a first object onto a second object with use of a projection exposure apparatus such as above, while scanning the first and second objects in synchronism with each other; and developing the second object having the pattern transferred thereto.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.